1. Field of Industrial Utility
The present invention relates to an objective lens drive unit.
2. Related Art
With optical disk systems that use optical disks as recording media, an optical pickup is used to write information signals onto the optical disk and to read them from the disk. The optical pickup uses an objective lens drive unit for allowing the laser beam to converge correctly in individual information pits on the optical disk. With the objective lens drive unit, it is necessary that the objective lens be subjected to precise position control two-dimensionally in both a focusing and a tracking direction. To insure that this two-dimensional following servo is performed in a precise manner, a construction for holding the objective lens at a specified neutral point (the stationary neutral position) is an important element that determines the performance of the optical pickup.
One of the means that have been proposed to hold the objective lens at the specified neutral point is by providing a magnetic restoring force, as described in Unexamined Published Japanese Utility Model Applications (kokai) Sho 58-179635, Sho 58-163908, Unexamined Published Japanese Patent Application (kokai) Sho 62-141646 or the like.
The objective lens drive units described in these patents that are designed to provide a magnetic restoring force have had the problem that they are limited in the range over which magnetic restoring forces can be created and that some improvement is necessary such as the use of more than one magnetic piece. Another problem is that one magnetic piece is capable of holding the objective lens at the neutral point in either a focusing or a tracking direction and that in order to insure that the objective lens is held in both focusing and tracking directions, a two-phase magnetic piece is necessary, which will increase the complexity of the structural design and the cost of the equipment.
With a view to solving these problems, the objective lens drive unit described in Unexamined Published Japanese Patent Application Hei 1-317234 has magnets mounted in a face-to-face relationship with drive coils and the magnets are magnetized with polarization in a certain direction, say, a focusing direction. One magnetic piece in the form of a vertically elongated thin sheet as indicated by 30 in FIG. 23 is placed within the magnetic circuit of each of said focusing magnet portions and a two-dimensional restoring force is generated in both the focusing direction and a tracking direction by the magnetic attraction acting on the magnetic piece 30, so that the objective lens is held at the neutral point.
Using a magnetic piece 30 with a width (xp) of 0.8 mm and a length (yp) of 4.6 mm, the inventors measured the resonant frequency fo of the moving part in the focusing direction (to be hereinafter referred to as "focusing fo") and the resonant frequency fo of the moving part in the tracking direction (to be hereinafter referred to as "tracking fo"). The results were as follows: focusing fo=26.6 Hz; tracking fo=17.0 Hz. With compact disks (CDs) and the like, the values of focusing fo and tracking fo are typically around 20 to 30 Hz. Further, their values depend on the width (xp), length (yp) and thickness (tp) of the magnetic piece 30 and optimal values of resonant frequency (fo) can be attained by properly varying those dimensions.
With recent models of CD-ROM and other optical disk systems that are capable of operation in a quick mode 2 to 4 times as fast as in the normal mode, a growing need has arisen to increase the tracking fo while suppressing the changes in the focusing fo. However, the magnetic piece 30 is provided within the magnetic circuit of each of the focusing magnets and if its shape is varied by changing its width (xp) or length (yp) or thickness (tp), not only the tracking fo but also the focusing fo will change, thus failing to meet the need for increasing the tracking fo while suppressing the changes in the focusing fo.
A further problem is that in order to increase the tracking fo without changing the shape of the magnetic piece 30, its length yp must be increased but then the magnetic circuit of the focusing magnet portion is upset to degrade the linearity of its characteristics. There has also been the problem that the thickness of the optical pickup increases to such a level that it is no longer compact.
The objective lens drive unit described in Unexamined Published Japanese Patent Application (kokai) Hei 1-317234, supra, has had the following additional but serious problem. In a structure that allows the magnetic piece to be moved relative to a fixed magnet, the converging position of the magnetic piece in the tracking direction (which is generally referred to as the "stationary neutral position") is positioned to lie in a face-to-face relationship with the design neutral position of the magnet where the highest flux density is achieved (which coincides with the position of the middle portion in the peripheral direction) so that the magnetic attraction acting on the magnetic piece will be maximal in that position, thereby insuring that the objective lens is held in a stable manner. In practice, however, imbalanced flux, leakage flux, variations in workmanship and other factors make it difficult for the magnetic piece to be held stable in the design neutral position.
Namely, when the magnetic piece is placed in the position which is shifted from the designed neutral position, the center of the objective lens is shifted from the optical beam under the rest condition. This causes to the shape of the optical beam spot to buckle. Moreover, in the rest condition, it is impossible to obtain the information from the optical disk although the information from the optical disk is obtained in the normal condition. This becomes the problem in the productivity.
Moreover, in the consideration of the characteristic of the actuator of the objective lens drive unit, the neutral position is designed. However, in the shifted condition described above, the magnet pieces are tilted in one side. In the tilted side, the area where the drive unit is driven in the stable condition is limited.
This problem is significant not only in the tracking direction but also in the focusing direction.